Recording performance for magnetic disks is commonly determined by three basic characteristics--PW50, overwrite, and noise. PW50 is the pulse width of the bits expressed in either time or distance, defined as the width of the pulse at half-maximum. A narrower (and more well-defined) pulse allows for higher recording density. A wide PW50 means that the bits are crowded together, resulting in each adjoining bits interfering with one another. This interference is termed inter-symbol interference. Excessive inter-symbol interference limits the packing density of bits in a given area, and hence limits the recording capacity of the magnetic media.
Means of reducing PW50 include reducing the magnetic remnant-layer thickness product ("Mrt") or reducing the thickness ("t") for a given alloy of the magnetic layer of the media (the media being comprised of a substrate, and one or more layers, including a magnetic layer, formed on the substrate). Another means of reducing PW50 is to increase hysteresis loop squareness ("S", including coercivity squareness, "S*", and remnant coercivity squareness, "S*rem"), and narrow the switching field distribution, as described by William and Comstock in "An Analytical Model of the Write Process in Digital Magnetic Recording," A.I.P. Conf. Proc. Mag. Materials, 5, p. 738 (1971). Yet another means for reducing the PW50 is to increase the coercivity ("Hc") of the media.
Overwrite ("OW") is a measure of the ability of the media to accommodate overwriting existing data. That is, OW is a measure of what remains of a first signal after a second signal (for example of a different frequency) has been written over it on the media. OW is poor when a significant amount of the first signal remains. OW is generally affected by the coercivity, the thickness, and the hysteresis loop squareness of the magnetic layer. For example, a high Hc can result in a recording head having difficulty saturating (i.e., writing data to) the magnetic layer, resulting in poor overwrite.
Noise performance of a magnetic layer can be defined in terms of read jitter and write jitter. Read jitter is primarily determined by the amount of signal available from a bit, and the electronic noise in the channel. A higher Mrt or thicker magnetic layer for a given alloy will typically provide reduced read jitter. Unlike read jitter, write jitter is determined by the intrinsic noise of the magnetic layer. Intrinsic media noise has been theoretically modeled by Zhu et al. in "Micromagnetic Studies of Thin Metallic Films", J. Appl. Phys., vol. 63, no. 8, p. 3248 (1988), which is incorporated by reference herein. Chen et al. describe the source of intrinsic media noise in "Physical Origin of Limits in the Performance of Thin-Film Longitudinal Recording Media," IEEE Trans. Mag., vol. 24, no. 6, p. 2700 (1988), which is also incorporated by reference herein. The primary source of intrinsic noise in thin film media is from interparticle exchange interaction. In general, the higher the exchange interaction, the greater the noise.
The noise from interparticle exchange interaction can be reduced by isolating the individual particles. This may be accomplished by spacing the grains apart from one another, or by interposing a non-magnetic material or insulator at the grain boundaries as described by Chen et al. in the aforementioned "Physical Origin of Limits in the Performance of Thin-Film Longitudinal Recording Media." The amount of separation need be only a few angstroms for there to be a significant reduction in interparticle exchange interaction. A reduction in interparticle exchange interaction has also been tied to an increase in Hc by Chen et al. and by Zhu et al. in the aforementioned references.
There is another interparticle interaction, called magnetostatic interaction, which acts over a much greater distance between particles as compared to the exchange interaction. Reducing the magnetostatic interaction does reduce intrinsic media noise slightly. However, the effects of magnetostatic interaction actually improve hysteresis loop squareness and narrow the switching field distribution, and hence improve PW50 and OW. Therefore, magnetostatic interaction is generally tolerated.
In peak-detection type recording channels, noise, together with inter-symbol interference, contributes to the uncertainty in the location of the individual bits, which cause the data to be read with some displacement in timing from that which is expected. This displacement is referred to as bit shift. Bit shift must be minimized for a given timing window of a bit in order to assure accuracy in reading that bit.
In order to obtain the best performance from the magnetic media, each of the above criteria--PW50, overwrite, and noise--must be optimized. This is a formidable task, as each of these performance criteria are inter-related. For example, obtaining a narrower PW50 by increasing the Hc will adversely affect overwrite, since increasing Hc degrades overwrite. A thinner media having a lower Mrt yields a narrower PW50, however the read jitter increases because the media signal is reduced. Increasing the squareness of the hysteresis loop contributes to narrower PW50, but generally increasing squareness increases noise. Thus, the amount that PW50 may be narrowed is limited by the increase in noise.
Providing a mechanism for separating or isolating the grains to break the exchange coupling can effectively reduce the intrinsic media noise and increase coercivity. Noise is improved by eliminating the interparticle exchange interaction. A slight further reduction of noise is possible by reducing magnetostatic interaction, but this reduces the hysteresis loop squareness and increases the switching field distribution, which degrades PW50 and overwrite.
In order to obtain the optimum media performance, the Mrt must be reduced for better overwrite and PW50, but still retain sufficient signal to maintain acceptable read jitter. This is principally accomplished by reducing the magnetic layer thickness (thereby reducing the space loss between the recording head pole tip and the magnetic layer), and using an alloy having a higher saturation magnetization ("Ms").
Therefore, an optimal thin film magnetic recording media for high density recording applications, i.e., that can support high bit densities, provides low noise without sacrificing the switching field distribution and squareness. Recording density can then be increased since bit jitter is reduced. In order to achieve optimal performance, the individual grains of the magnetic layer of the media must be isolated to eliminate the exchange interaction, and grains must be uniform and have a tight distribution of sizes to minimize intrinsic media noise while maintaining high coercivity and squareness.
One type of magnetic media which has allowed optimizing certain of the above performance criteria is based on alloys of cobalt (Co) and platinum (Pt). CoPt is typically alloyed with nickel (Ni), chromium (Cr), etc. Attributes of CoPt alloys have been described by Murdock et al. in "Roadmap to 10 Gb/in.sup.2 Media: Challenges", IEEE Trans. Mag., 1992, page 3078, by Opfer et al. in "Thin Film Memory Disk Development", Hewlett-Packard Journal (Nov. 1985), and by Aboaf et al., in "Magnetic Properties and Structure of Co--Pt Thin Film", IEEE Trans. Mag., page 1514 (1983), each incorporated herein by reference. Increasing storage capacity demands and performance requirements have motivated a search for ways to improve Co--Pt based alloys.
In order to control coercivity, it is known to sputter deposit the magnetic layer in the presence of gaseous oxygen or nitrogen, for example as taught by Yamashita et al. in U.S. Pat. No. 4,988,578, which is incorporated herein by reference. Also, in order to decrease the media noise, it is known to introduce oxygen into the magnetic layer in a concentration of 5 to 30 at. %, as taught by Howard et al. in U.S. Pat. No. 5,066,552, incorporated by reference herein. As with the teachings of Yamashita et al., Howard et al. teaches the formation of the magnetic layer of the disk by vacuum sputtering in an argon ambient into which oxygen has been introduced. According to these references, either oxygen or nitrogen is introduced into the magnetic layer from the ambient.
Shimizu et al. in "CoPtCr Composite Magnetic Thin Films", IEEE Trans. Mag., vol. 28, No. 5, page 3102 (1992), which is incorporated herein by reference, disclose co-sputtering an oxide of an impurity and the CoPtCr magnetic alloy. Specifically, the media noise, in-plane coercivity, and S* were noted to improve with an introduction of approximately 10% by volume ("vol. %") SiO.sub.2. This result is presumably obtained because the immiscibility of the oxide causes it to segregate at the magnetic grain boundaries, thereby reducing the intergranular exchange interaction (coupling) between the magnetic grains.
A variation on the approach of Shimizu et al. is disclosed by Howard in U.S. Pat. No. 5,062,938, which is incorporated herein by reference, in which an impurity such as yttrium (Y), silicon (Si), hafnium (Hf), germanium (Ge), tin (Sn) or zirconium (Zr) is co-sputtered with the cobalt-based magnetic alloy material. Thereafter, the sputtered layer containing the impurity is oxidized when annealed separately in the presence of oxygen. It is believed that the impurity elements will segregate at the grain boundaries, and when oxidized, break the exchange coupling between the grains.
There are a number of disadvantages to the introduction of impurities as taught by these references. First, the addition of an impurity material (e.g., 10% by volume SiO.sub.2 by Shimizu et al. and 12 atomic percent Y by Howard) results in an excessive decrease in Ms. Therefore, the thickness of the magnetic layer must be increased to maintain Mrt. This is undesirable because an increase in layer thickness is generally accompanied by an increase in space loss between the head and the media which results in broad PW50 and poor overwrite. Second, the sputtering process is made more complex and more costly by the requirement that additional materials be sputtered. The additional step of oxidizing a sputtered layer also adds to the complexity and cost of the sputtering process. Third, introduction of oxygen or nitrogen from a gas in the sputtering environment is a difficult process to control precisely, and variations in oxygen or nitrogen concentrations from disk to disk through sputtering is not uncommon. This is particularly true for individual-disk sputtering systems such as those manufactured by Intervac Corp. and others.
Therefore, there is at present a need in the art for a method of reducing media noise without jeopardizing other media performance characteristics such as high coercivity, high degree of squareness, high SNR, high overwrite, and low PW50.